Semiconductor gas sensors have been widely used in various applications such as measurement of drivers' blood alcohol levels, detection of explosive gases, detection of exhaust gases from automobiles, and detection of toxic industrial gases due to their advantages of high sensitivity, miniaturization, integration, simple operating circuits, and economical prices. With the recent advances in high-tech industries and growing interest in human health and environmental pollution, there has been a rapidly increasing demand for gas sensors for the detection of indoor/outdoor environmental gases, gas sensors for self-diagnosis of diseases, and high-performance artificial olfactory sensors mountable on mobile devices. Thus, there is also a rapidly growing need for oxide semiconductor gas sensors that are highly sensitive and fast respond to very low concentrations of analyte gases.
A remarkable improvement in the sensitivity of gas sensors is a main requirement for accurate detection of very low concentrations of harmful gases, explosive gases, and environmental gases. The operating principle of a gas sensor depends on changes in charge concentration in an oxide semiconductor through interaction between an analyte gas and the surface of the oxide semiconductor. Under these circumstances, three possible approaches are suggested to improve the sensitivity of gas sensors: (1) an approach to maximize gas adsorption by using nanoparticles with high surface area/volume ratio; (2) an approach to increase the proportion of electron depletion layers (in the case of n-type semiconductors) or hole accumulation layers (in the case of p-type semiconductors) present around the surface using nanoparticles; and (3) an approach to design a nanoporous structure of a sensing material such that an analyte gas can be supplied over the entire surface of the sensing material.
For the approaches (1) and (2), it is particularly advantageous to use nanoparticles whose size is on the order of several nanometers (nm). However, in very small nanoparticles, the Van der Waals attractive force increases considerably in inverse proportion to the particle size, and as a result, most of the nanoparticles tend to form dense secondary agglomerates. Thus, gas sensing reactions occur at or near the surface of the secondary particles and gas diffusion into the inner part of the secondary particles requires a long time, making it difficult to obtain high sensitivity and leading to very slow sensing.
In this connection, a porous gas sensing unit of a semiconductor gas sensor and a production method thereof are known (Patent Document 1). According to this method, an alumina slurry is coated on a polyurethane sponge and sintered to prepare a porous alumina ceramic from which the polyurethane sponge is removed, and a paste of a SnO2-based compound as a gas sensing material is coated and dried thereon to produce the porous gas sensing unit. A gas sensor including a gas sensing layer composed of In2O3 having a nanoporous hollow structure or nanoporous hierarchical structure and a method for fabricating the gas sensor was reported (Patent Document 2). Further, according to a method described in Patent Document 3, nanoporous tin oxide nanotubes are prepared by coating anodic aluminum oxide templates with a surfactant containing amine groups, filling tin oxide/titanium oxide nanoparticles having carboxyl groups in the templates, inducing the formation of peptide bonds between the surfactant and the nanoparticles, followed by a serious of subsequent processing steps, such as removal and sintering of remaining nanoparticles, coating with an electrode material, and etching. Furthermore, many research groups have reported various techniques for synthesizing nano-hierarchical structures and have proposed the fabrication of high-performance gas sensors using the nano-hierarchical structures that permit smooth entrance and exit of gas and have large specific surface areas (Non-Patent Document 1).
In attempts to increase the sensitivity of oxide semiconductor gas sensors, numerous studies have been conducted to increase the access of analyte gas to nanostructures. For example, nanostructures such as nanoparticles, nanowires, nanorods, nanosheets, and nanocubes and nano-hierarchical structures in which the nanostructures are combined and bound to form other types of high-dimensional structures have been investigated as sensing materials (Non-Patent Documents 2-5). Particularly, nano-hierarchical structures reported in Non-Patent Document 1 are advantageous for use in gas sensors because they have many pores for high gas accessibility while maintaining their large specific surface areas.
However, most of the conventional techniques are associated with the preparation of nanostructures by hydrothermal synthesis or solvothermal synthesis of solutions of raw material salts and the fabrication of gas sensors based on pores naturally formed in the course of the preparation of the nanostructures. Accordingly, it is impossible to control the size and shape of the basic nanostructures and the pore size, shape, and volume of the nanostructure-bound states because nucleation, nanostructure growth, and self-assembly between the nanostructures occur naturally in solutions at high temperature and high pressure.
Independent and accurate control of nano-, meso-, and macro-scale pores is of great importance for the design of gas sensors because the diffusion mechanisms of analyte gas are very sensitively dependent on the size, distribution, volume, etc. of pores. For example, surface diffusion becomes dominant in nanopores having a size of several nm, Knudsen diffusion considering collisions of gas with the outer walls of pores occurs in meso-scale pores having a size of ˜5-50 nm, and normal diffusion considering only collisions between gas molecules occurs in macropores whose pore size is 100 nm or more (Non-Patent Documents 6 and 7).
There is thus an urgent need to develop a technique for directly and elaborately designing the type, size, and density of pores in sensing materials that have a direct influence on the improvement of gas sensing characteristics and simultaneously functionalizing the sensing materials with pores of different sizes, achieving ultra-high sensitivity to analyte gas.    Patent Document 1: Korean Patent Publication No. 10-2003-0003164    Patent Document 2: Korean Patent Publication No. 10-2010-0025401    Patent Document 3: Korean Patent Publication No. 10-2011-0115896    Non-Patent Document 1: J.-H. Lee, Sens. Actuators B 140 (2009) 319-336    Non-Patent Document 2: H. Zhang, 17 (2007) 2766-2771    Non-Patent Document 3: Q. Dong, Nanotechnology 17 (2006) 3968-3972    Non-Patent Document 4: P. Sun, Sens. Actuators B 173 (2012) 52-57    Non-Patent Document 5: W. Guo, Sens. Actuators B 166-167 (2012) 492-499    Non-Patent Document 6: M. Tiemann, Chem. Eur. J. 13 (2007) 8376-8388    Non-Patent Document 7: T. Wagner, Chem. Soc. Rev. 42 (2013) 4036-4053